Much of condensed matter physics addresses weak excitations of materials and
devices from their ground states. More often than not the complications caused
by electron correlations can be captured by the concept of approximately
non-interacting quasi-particles with well-defined dispersion relations.
Quasi-particles are not eigenstates and eventually decay on characteristic
time scales that depend on material and environment. In conventional electric
insulators, the lowest energy excitations are lattice waves with associated
bosonic quasi-particles called phonons. The elementary excitations of magnetic
order are spin waves and their bosonic quasi-particles are the
“magnons” [1]}. In magnetic
insulators, magnons and phonons coexist in the same phase space and can form
hybrid quasi-particles that may be called “magnon
polarons”. Yttrium iron garnet (YIG) is the material of
choice to study magnons, phonons, and magnon polarons because of their
longevity in YIG bulk and thin-film single crystals. For more than half a
century, Professor Sergio Rezende and his team seminally contributed to our
understanding of magnons, phonons, and their hybrids by experimental and
theoretical research, see e.g. [2]}, [3]}. Recently we
extended Prof. Rezende's Boltzmann theory of the spin Seebeck effect
[4]} in magnetic insulators by introducing “ferrons”, i.e. the excitation of the electric dipolar order
in ferroelectrics (FEs), and explored their ability to transport heat and
polarization, including the associated Seebeck and Peltier effects
[5]}, [6]}. Here we expound the analogies and differences
between magnets and FEs in their ground states [7]} as well as in
their dynamic and transport properties, thereby comparing the magnonics of
ferromagnetic insulators with the “ferronics” of ferroelectric insulators.
